The present invention relates to a control blade for use in a nuclear reactor which is adapted to be inserted into and extracted from a nuclear reactor core for the purpose of controlling the power of the nuclear reactor. More particularly, the invention is concerned with a long-life flux-trap type control blade suitable for use in a boiling water reactor (BWR).
In general, a control blade for use in a boiling water reactor has a central tie rod and a plurality of wings formed by U-shaped sheath plates attached to the tie rod, each wing containing a multiplicity of neutron absorber rods. Each neutron absorber rod is composed of a clad tube made of a steel such as stainless steel and grain of boron carbide (B.sub.4 C) charged in the clad tube. In order to prevent the grain of boron carbides from moving freely within the clad tube, partition balls are placed at a predetermined interval within the clad tube.
The boron carbides in the form of grain charged in the neutron absorber rod progressively decreases its neutron absorption power (capacity) due to absorption of neutrons, and generates He gas as a result of reaction between boron-10 (.sup.10 B) and neutrons resulting in a rise of the pressure within the clad tube. The lifetime of the control blade determined by the neutron absorption power is referred to as "nuclear lifetime", while the lifetime determined by the internal gas pressure of the clad tube is referred to as "mechanical lifetime".
The control blade, which is adapted to be inserted into and extracted from the nuclear reactor core, is not uniformly exposed to neutrons. For instance, the rate of neutron exposure rate is high at the side edges and upper end of each wing. This means that these portions of the control blade absorb greater amounts of neutron than other portions of the control blade and, therefore, the nuclear lifetime is reached earlier in these portions than in other portions of the control blade. In consequence, the control blade has to be disposed of as a radioactive waste, even though sufficient lifetime is left in other portions thereof.
In order to obviate this problem, the present inventors have developed an improved control blade in which long-life neutron absorbers are disposed in the vicinity of side edges of wings where the degree of neutron exposure is high, as disclosed in Japanese Patent Laid-Open No. 74697/1978. This improved control blade, however, is still unsatisfactory from the view point of prolongation of lifetime of control blades, because it exhibits a lifetime which is only twice as long as that of ordinary control blades containing B.sub.4 C.
In order to cope with the demand for prolongation of lifetime of control blades, the present inventors have developed a long-life control blade capable of operating much longer than the above-mentioned improved control blade. This long-life control blade has, as disclosed in Japanese Patent Laid-Open No. 55887/1983, solid neutron absorption plates made of a long-life neutron absorber and disposed in each wing thereof. The neutron absorption plate has apertures or recesses whose sizes and distribution are so determined that the amount of material removed by the presence of such apertures or recesses is comparatively small in the portion where the axial distribution of the shut down margin is small and is comparatively large in the portion where the axial distribution of the shut down margin is large.
This long-life control blade, however, suffers from the following disadvantage, due to the use of hafnium (Hf) sheet as the neutron absorber. Namely, hafnium is expensive and has a large specific gravity (13.3 g/cm.sup.3) so that the cost and the weight of the control blade are increased undesirably. The increased weight of the control blade in turn requires a design of a control rod drive mechanism which can safely operate such heavy control blades because conventional control rod drive mechanism cannot withstand such heavy weight of the control blades.
The inventors, however, have confirmed that there still is a margin for the removal of material in the hafnium sheet which is used as long-life neutron absorber for the purpose of reducing the weight of the hafnium sheet, and that ordinary control blade drives are still usable provided that the weight of the control blade is reduced by the removal of the material.